Thứ Sáu, 24 tháng 7, 2009

Phương pháp địa vật lý trong khảo cổ học

Đi khảo cổ thật là VUI!
(Minh họa lấy từ "Hành trình vào KCH Việt Nam của Reinecke.A.)


by Lambert Dolphinformerly Senior Research PhysicistSRI International, Menlo Park, California

World War One brought the discovery that photographs behind enemy lines taken from airplanes could be of great value in warfare. Not longer after this, observers taking random photographs from the air over rural England noticed that traces of old Roman walls, forts and roads could be seen on aerial photographs but otherwise went unnoticed under cornfields and pastures when archaeologists wandered about the countryside on foot. Terrain photos from captive balloons had been made even earlier (1860) but it was only in the 1930's and 40's that archaeologists began to take advantage of photos from the air over archaeological sites. Today, of course, stereo-pair color and color infra-red film photographs (or even the newer multi-spectral imaging methods) from the air, are the place to begin in mapping and understanding an archaeologically interesting area.
Prior to the Second World War electronic methods began to be employed in earnest in searching hr oil and large mineral deposits beneath the surface of the earth. Because of the big economic payoff, successful discoveries made possible by even primitive geophysical methods were high enough that R&D budgets soon became generous. An explosion of knowledge in geology, earth science, geophysical and remote sensing followed. After World War 2 all the sophistication brought by war time research then also became available to private industry, producing a new, even bigger, boom in geophysical exploration. Historically, the scale of exploration required for oil and mineral exploration for most of these methods was very large (of the order of kilometers), while in contrast the scale of interest to an archaeologist is only centimeters or meters. In addition to highly evolved aerial photography, airborne and satellite multi- spectral imaging instruments, good ground based geophysical instruments began to be commercially available in the 30's taking advantage of various physical phenomena.

Some basic geophysical methods include the following: (1) Seismic Reflection & Refraction, (2) Gravity, (3) Magnetics, (4) Electrical, and (5) Radioactivity. Method (1) is commonly used in oil exploration, engineering geology, and regional geology studies. The gravity method (2) is especially useful in oil exploration. Methods (3) and (4) find common application in mineral exploration, oil exploration, and regional geology studies. Finally radioactive methods are used in exploration for radioactive minerals. Common geophysical instruments and methods include:
· Earthquake studies (study of the interior of the earth on a large scale).
· Fibrosis (artificially shaking the ground at low frequencies).
· Explosion waves with Georgine listening (determines travel time, bending, dispersion, and reflection of low frequency sound waves).
· Gravimeter (measures variations in the earth's gravity field attributable to sub- surface features).
· Magnetometer (measures variations in the magnetic field of the earth due to sub-surface features, especially ferrous materials).
· Resistivity Mapping, Induced Polarization Mapping, Electrical Potential Measurements, Earth Currents (mapping electrical conductivity in the earth).
· Metal detectors (induced electrical fields).
· Geiger counter and Scintillation counter (natural radioactive decay of certain earth materials).
· Neutron Activation (artificially induced radioactive emission).

As mentioned, the application of some of the above geophysical methods to archaeology began in earnest after World War II, but in contrast to the huge budgets available for petroleum and mineral exploration, archaeological budgets have almost always been minuscule. Usually the chief archaeologist at a site is a reputable and experienced professor whose modest salary is paid by his school so that he can teach university classes and do some seasonal field research on the side. The field work in archaeology has always depended mostly on student volunteers and assistants. Small amounts of financing are sometimes available from museums or grant institutions such as National Geographic Society, the National Science Foundation or the Smithsonian Institution. Usually digging at an archaeological site must be done by hand though occasionally massive amounts of overburden must be removed, or trenching done, with the help of a back-hoe or bulldozer. Cataloging, preserving artifacts (conservation), and publication of scientific papers occupies the off-season, but often funding levels for these important activities are also minimal. But even with the limited budgets archaeologists have with for decades, geophysical methods can be of great value to an archaeologist. Some of these reasons include:
· Archaeology is destructive. As a site is dug up it is systematically destroyed, Hence each step of the dig must be painstakingly slow with careful documentation at each level. Geophysical probing on the other hand is rapid, non-destructive, and does not disturb the site.
· Not all archaeological sites can be excavated. Examples would be historic buildings, churches, mosques, the pyramids, parks, and areas which underlie modern urban development. Again, geophysical methods are non- destructive and very rapidly employed, hence often cost effective in the long run. In some cases these methods may be all that the archaeologist is able to use at some sites.
· Archaeologists can be greatly helped in setting his digging priorities if geophysical methods can be used ahead of time. Geophysical surveying can in many cases reveal artifact-laden vs. barren ground, and disclose important underground features: buried walls, voids, tunnels, ancient streets, etc.
· Many decades may be required to explore a given site, such as a tell. In fact total excavation of a site may be impractical. Geophysical survey work at a given sites can usually be done in a few days or weeks of effort, the results of which are useful for many years of subsequent excavation work.
· Salvage archaeology has become important as urban sites encroach on archaeological sites in many parts of the world. Thanks to modern legislation, substantial funding for archaeological research prior to the clearing of an area and construction of new buildings may be available. In many such cases, however, the time available for the archaeological effort may be very limited. Geophysical methods may be of great value as the site will often be totally destroyed by the new construction.
Ground Penetrating Radar (GPR) was invented in the 1970's, originally for military purposes such as locating land-mines and underground military tunnels. Soon public utility companies began to be keenly interested in such radars in hopes they would provide a practical method for mapping pipes and utility lines under city streets, and for locating cavities and voids. Most recently radars of this type have been used from aircraft for mapping the surface of the earth through jungle or forest cover. GPR technologies have proven to be of great usefulness in archaeology, especially in Israel. Radar from the air is seldom of use to the archaeologist these days except for large sites covered by jungle such as are found in the Yucatan or Central America. Foliage-penetrating radars are now used widely for topographic mapping of the land surface beneath jungle canopy and forest cover. Thermal-infrared imaging methods measure the surface temperature of the earth to an accuracy of a fraction of one degree. The electronic scanning equipment necessary for such measurements was originally available only to the military and the instruments cost from $100,000 to $1,000,000. In recent years portable instruments of great sensitivity have become commercially available at greatly reduced prices. These instruments can be used from a tripod on the ground, or from helicopter or airplane by viewing through a hole in the fuselage. Borehole Technology. Radars, seismic and resistivity other probes are often lowered into holes drilled into an archaeological site, to permit geophysical probing at depth. Core drill soil samples can be a big help in identifying the various historic levels and strata at a layered archaeological site such as a tell. When chambers or voids are encountered while drilling, these can be explored (and video taped) using a down-hole television camera equipped with lights. Holes drilled into an archaeological site are obviously much less damaging than trenches or tunnels and they can either be filled or capped after use. Not all individuals or companies who offer geophysical assistance to the archaeologist are reputable or professionally competent. Fraudulent self-made experts---whose instruments may be little more than electronic water dowsing rods-commonly offer services that are of little value. Some geophysical instruments on the market may promise amazing results in identifying metals at great depth by type and quantity but many of these operate by methods unknown to reputable science. Geophysical records, even when made using legitimate instruments, are also of little value unless the data is collected and interpreted correctly. Archaeologists should not expect his geophysicist to work wonders for him at all sites. In some cases a combination of instruments may be appropriate, in other cases no known method may prove really very useful or cost effective. The following legitimate geophysical methods and instruments are in use in the service of archaeology today:
A wide variety of "metal detectors" are commercially available today; they have the advantage of being easy to use and most cost only a few hundred dollars. The larger the search coil, the deeper the penetration; however coins and small metal objects can be detected only a few inches deep and very large metal objects only to depths of a few feet. Non-metal objects are not detected. Some areas are too "noisy" for metal detectors. "Noise" can originate from power lines, or from obscuring signals caused by nearby parked cars, scattered nails, re-bar or metallic litter at the site. Highly mineralized areas are difficult to work in, and certain rocks such as iron-rich basalt can be troublesome for metal detector work. Metal detectors are "active" instruments. A battery-powered transmitter in the unit radiates a relatively low-frequency alternating current signal into the ground by means of a transmitting coil. If the signal from the transmitter encounters any type of conducting metal or mineral in the ground an induced current flows in the subsurface target. This induced current then re-radiates a weak signal back to the surface. The latter signal is out-of-phase with the transmitted signal and thus is easily detected by a receiving coil. Modern metal detectors have circuitry for carefully balancing out any direct signal leakage between transmitter and receiver coils and for discriminating between large and small, shallow or deep, and ferrous or non-ferrous metals. The simpler instruments of this type are useful for "coin shooting" at old ghost town sites, or archaeological sites (on land or under the sea), and for locating gold or silver deposits within a quartz vein in a lode mine. Small objects such as coins usually must lie within a few inches to a foot of the surface to be detected by metal detectors. The sensitivity of metal detectors is a steep function of the coil diameter, however with large coils and ample transmitter power larger metal objects can be located to depths of 10 or 15 feet using metal detectors. Claims for detection at greater depths as well as identification of metals by type are suspect.
The resistivity method of subsurface exploration is powerful but often tedious to employ unless an automated instrument is available. The method is simple: Current is introduced into the ground through one pair of electrodes. Current flow between these electrodes fans out through the ground in a pattern and intensity that depends on the conductivity of the ground and any stratification or obstacles that lie in the vicinity of the electrodes. A second pair of electrodes is then used to quantitatively measure the voltage pattern on the surface resulting from the current flow pattern of the first set of electrodes. A number of different electrode configurations are used in practice, but in simplest form the operator takes measurements along a straight line ("traverse"), moving his electrodes in pairs. He then repeats the measurements along a parallel line until the area of interest has been covered with a rectangular grid of electrode positions. If multiple electrodes are used and the results recorded automatically at the push of a button, the area to be examined can be searched more efficiently, and also probed at various depths at the same time. (As a rule of thumb, the depth of maximum sensitivity for resistivity sounding is about 1.5 times the electrode spacing in typical arrays). A crew of two can easily study an area of perhaps 1000 square meters in a day. Typical electrode spacings might be 0. 3 to 1.0 meters for shallow targets. Once the resistivity data has been collected, a simple computer program quickly generates a three-dimensional map of ground electrical resistivity or conductivity. Targets most easily seen on resistivity surveys are cavities or voids, but buried walls and filled trenches can often be mapped. The target depth divided by the diameter of the target should be less than 3 or 4 for best sensitivity, though some experts claim to be able to detect targets with a depth to diameter ratio of 9 or more. Boulders, geological stratifications and water-table depth can also be successfully located by the use of resistivity by selecting appropriate electrode spacing to allow the probing current to enter the ground to the appropriate depth. Resistivity meters employed in oil prospecting are often powered by large generators using very high voltages and electrodes spaced perhaps hundreds of meters or kilometers apart, but instruments suitable for archaeological use are battery powered, easy to use, and usually priced under $1500. Resistivity instruments no different than those used by professional geophysicists, but with fancy labels attached, are often found advertised for five times the price of standard instruments. Let the buyer beware!
Radars designed for probing into the earth typically operate from 30 to 300 MHz-the frequency being determined by the length of the dipole antennas used. It is necessary to use relatively low frequencies because the earth almost always is a good absorber of radar waves. Unfortunately, low frequencies imply long probing wavelengths and long wavelengths imply low resolution. A very short pulse is used allowing accurate measurement of depth to the target, however the antenna beam is very broad (90-120 degrees usually) and can not easily be narrowed because the antennas become too big and bulky. Very often GPRs are mounted on a small wheeled cart which is hand towed across the area of interest, that is, if the search area is reasonably flat and relatively free of brush and boulders. The echoes are displayed in a continuous strip oscilloscope false color record for ease of interpreting results. In recent years the state of the art in GPR technology has been greatly improved by computer signal processing methods, since the performance of these radars is almost always "clutter limited." Clutter signals are unwanted reflections, off-axis echoes, and multiple scattering echoes. These signals obscure the target of interest under bands of signals but in many cases digital processing improves radar performance by many orders of magnitude. When cart-mounted radar can be used, an experienced operator can often traversing large areas of surface at a site in a single day. The radar output can be recorded on a standard home video tape for archiving and detailed study, and also printed out on strip-chart paper for immediate on site analysis. GPRs are usually limited not only by clutter but also by attenuation of the radar signal in the soil. This is most severe in clay soils and damp soils where the salt content is high. The depth of penetration at some sites may be less than 1 foot, or under favorable conditions, many tens of feet or even hundreds of feet. Commercial cart GPRs are priced from about $18,000 to $40,000 and operator training and experience is necessary to interpret the records. Very often cart radars can not be used because of rugged surface terrain. Or perhaps the area to be explored is underground---inside a tunnel or cistern or along a confined area such as a hillside. Portable individual transmitting and receiving dipoles are useful in such cases. But the data must now be recorded point by point, usually by taking Polaroid photos. Targets of interest can be triangulated and mapped if these targets can be viewed from various aspect angles. Portable GPRs are well suited for discovering cavities and voids, and when soil attenuation values are low they can detect caves, tombs, or chambers one hundred feet or more in depth. Interpretation of GPR records of all types is unusually difficult requiring operator skill and experience for satisfactory results.
Sound waves are not easily coupled into soils, except at very low frequencies (a few Hertz, or cycles per second) but at higher frequencies sound waves can be used in rock or solid walls as a helpful diagnostic tool. Frequencies used for probing in bedrock or stone are generally 1000 to 30,000 Hertz (cycles/second). A coupling gel, or mud layer, is necessary to couple the seismic signal into and out of the transmitting and receiving transducers and this makes field measurements somewhat time consuming unless only a few locations are to be surveyed. High-frequency sounding is especially useful for finding tombs and voids in areas of high radar signal absorption. For example, the Valley of the Kings in Egypt has very high radar attenuation, but the same limestone can be probed with high-frequency sound waves for distances well beyond 100 feet. Measuring the thickness of a wall or pillar is readily done with this method. High-frequency seismic sounding instruments are not presently commercially available, but can be custom built for about $10,000.
The earth's magnetic field is slightly disturbed by some kinds of archaeological anomalies such as fired clay pottery. The magnetic signals associated with archaeological features are very small and easily obscured by trash metals, power lines, nearby automobiles, and the like. Magnetometers are most suited for remote, isolated sites away from modern buildings and debris. Magnetometers cost from about $1500 to $10,000 and can be used in pairs (a "differential magnetometer" to subtract out all but the wanted signals. Modern magnetometers are sensitive to field changes of about 1 gamma-the earth's weak magnetic field intensity is of the order of 50,000 gammas. Magnetometry has been successfully used to located imported stone at some well-known archaeological sites. Fired mud brick has a reasonably high magnetic anomaly and of course ferrous materials such as one might expect at an Iron-Age or later site, give rise to very large magnetic anomalies.
Gravity is one of the weakest of all forces found in nature. Yet, the earth's gravity field is very slightly altered by such features as subsurface voids or caves. Suitable gravity meters, known as "microgravimeters" cost of the order of $50,000 and require a very experienced trained operator. Point by point measurements must be made, which may be time consuming. The data must be carefully corrected for such things as surface topography and diurnally varying "earth tides." For these reasons gravity surveys have been little used in archaeology to date.
Conventional aerial (stereo-pair) photos of a site are very useful, as has been suggested, since outlines and features not visible from the ground frequently show up in aerial photos. Thermal infra-red (IR) imagery requires a scanner, usually cooled by liquid nitrogen, (instrument cost $15,000 to 50,000), but surface temperature differences of a small fraction of one degree can be measured. At night radiation cooling of the ground is not uniform if there are subsurface features that impede or enhance heat flow. In additional to diurnal heating and cooling, seasonal heat flow temperature changes can often be detected providing information on deeper archaeological anomalies. Heat flow through rock and soil is very slow---rock is an excellent heat insulator---so infra- red measurements give information about temperatures near the surface, not about temperatures deep within the earth. In spite of the limitations, false-color images showing temperature contours can thus provide interesting clues for the archaeologist at some sites, especially if such measurements can be made carefully at periodic intervals through an entire year. The Temple Mount in Jerusalem is an ideal site for on-going thermal infra-red imaging studies and Tuvia Sagiv, an architect from Tel Aviv, has already obtained some fascinating thermal IR images of the Temple Mount area. These can all be done from a distance or from the air.
If an archaeological site is complex and important, likely to be excavated for many field seasons, geophysical methods can be most useful since they are non-destructive and rapid. The archaeologist can hope to chose digging priorities based on survey findings. Some sites (monuments or parks) contain sites or buildings that can not be disturbed at all, so geophysical sensing may provide the only means of studying the site. Advice from a geologist who is familiar with an area can be helpful also. A combination of geophysical methods can be helpful as each method has its strengths and limitations. Archaeology is a time-honored exacting scientific discipline which provides us with some of our best information on human history and the past. It is to be hoped that more opportunities and sources of funding will develop so that modern geophysical methods can assist the archaeologist even more frequently than has been possible in recent years.

Lambert Dolphin Library

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